Carbon lamps similar to that shown in Figure 281 are made in a number of different voltages. The lowest voltage that it is practically possible to make a carbon lamp for is three and one-half. A three-and-one-half volt carbon lamp is designed to be operated on small dry cells such as flashlight batteries. The E. M. F. of a dry cell is about one and one-half volts, but when three small dry cells of the flashlight type are connected in series and used to operate a lamp, their voltage "drops," and the available E. M. F. is only about three and one-half volts.
Four-volt carbon lamps are intended to be operated on large dry batteries or wet cells because they do not lose their voltage as quickly as small dry cells. The table below gives the voltage and candle-power of the various small carbon lamps which are carried in stock by most electrical dealers or supply houses:
MINIATURE CARBON BATTERY LAMPS
3.5 volts for flashlight batteries
4 volts. 2 candle-power
5.5 volts for flashlight batteries
6 volts. 2 candle-power
6 volts. 4 candle-power
8 volts. 4 candle-power
10 volts. 6 candle-power